Test Your Knowledge - July

Tobacco hornworm
Tobacco hornworm
Sturgis McKeever, Georgia Southern University, www.insectimages.org

Yes, this is a tobacco hornworm (Manduca sexta).

Many gardeners have visited their vegetable garden to find a tomato plant that looked healthy the previous day, but has since had its leaves almost completely stripped. When this occurs, there's a good chance that either the voracious caterpillar above or the related species below, tomato hornworm (Manduca quinquemaculata), is the likely culprit. With their coloring, they blend in with the leaves and often go unnoticed until significant defoliation has occurred. Oftentimes their piles of frass (dark green waste material) are noticed before the insect is found.

Tomato hornworm
Tomato hornworm
Whitney Cranshaw, Colorado State University, www.insectimages.org

While these two hornworms look very similar and are easily confused with each other, a keen observer will notice that the tobacco hornworm has seven diagonal stripes along each side of its body. In contrast, the tomato hornworm has eight V-shaped marks along each side of its body. These marks point toward the head. A pointy structure or "horn" is found on the last body segment on each hornworm. The horn is usually red on the tobacco hornworm, but black on the tomato hornworm.

Both tobacco and tomato hornworms are most commonly found feeding on leaves of tomato and tobacco, but will also feed on other plants in the nightshade family (Solanaceae), including Irish potato, eggplant, pepper and various weed species. Less commonly, they will feed on developing tomato fruit. The tobacco hornworm can be found from southern Canada to South America while the range of the tomato hornworm extends from southern Canada to the southern U.S. While both the tobacco and tomato hornworms are found in South Carolina, the tobacco hornworm is seen more commonly.

Thankfully, these hornworms have several natural enemies, including wasps and wasp parasitoids (parasites that kill their hosts). The braconid wasp (see below) is one of these highly beneficial organisms. This tiny wasp (less than 1/8″ long) inserts its eggs into the hornworms. Larvae (immature insect stages) hatch from the eggs and feed on the internal organs of the hornworm, essentially eating the hornworm alive.

A braconid wasp
An adult braconid wasp, Cotesia congregata
R.J. Reynolds Tobacco Company Slide Set, R.JU. Reynolds Tobacco Company, www.insectimages.org

When the wasp larvae are ready to pupate (change from larvae to adults), they chew their way out of the hornworm and spin cocoons such as those seen below.

A tomato hornworm that has been parasitized by braconid wasps, exhibiting cocoons formed by the braconid wasp larvae.
A tomato hornworm with the cocoons in which the braconid wasp larvae pupate to adults
R.J. Reynolds Tobacco Company Slide Set, R.JU. Reynolds Tobacco Company, www.insectimages.org

These cocoons make the parasitized hornworms easily recognizable. At this point the hornworms stop feeding, but may continue to live for several days. When parasitized hornworms are present, it is extremely important that insecticides are not used in order to protect the parasitoid wasps. If insecticide use becomes necessary for some reason, place the parasitized hornwoms in an open container where the parasitoid wasps can finish pupating and emerge as adults that will go on to search for new hornworms to parasitize.

J. McLeod Scott
Home & Garden Information Center

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.